No problems, that's pretty straight forward to achieve. To have it appear just above the checkout complete button - you could include it as part of your custom checkout fields, as that's where they're included.

You've already got some custom fields in the custom checkout fields configuration option - so you can simply include this below the code you've already got in the textarea and it'll appear between those fields and the checkout button:

Ah sorry for the misdirection initially, @fc_marija is right there, it should be a lowercase and, rather than uppercase as it was in my earlier post. I've corrected my post for anyone else who might stumble upon this post later on. Again, sorry for providing the incorrect code previously!